Understanding the relative roles of phenotypic plasticity and genetic differentiation in shaping plant phenological responses is essential for predicting forest tree responses to climate warming. Although spring and autumn phenophases respond sensitively to rising temperatures, the underlying regulatory mechanisms may differ substantially, particularly across environmental gradients. We investigated intra-specific phenological variation in Quercus petraea across the French Pyrenees using a multi-environment experimental framework. This approach integrated 15 years of in situ phenological monitoring with common garden (CG) and reciprocal transplant experiments spanning an elevational gradient. We quantified key phenological traits-leaf unfolding, leaf senescence, and growing season length-and applied generalized linear mixed models to evaluate the relative effects of temperature, provenance-level differentiation, and their interactions. Leaf unfolding showed strong temperature-associated plastic responses and comparatively limited provenance-level differentiation. In contrast, leaf senescence showed weaker and less consistent temperature responses. Growing season length increased under warmer conditions, largely reflecting the combined effects of earlier leaf unfolding and variable senescence timing. Our findings reveal phase-specific differences in phenological responses, with stronger temperature-associated plasticity in spring than in autumn phenology. These results highlight the need for long-term, multi-environmental data for understanding tree phenology under a changing climate and provide insights for developing adaptive forest management strategies that maintain adaptive capacity.

In 2023, more than half of olive harvests ( Olea europaea ) across Spain, Greece, and Türkiye were lost to drought. The same year late freeze destroyed 90% of the peach crop ( Prunus persica ) on the Georgia Piedmont and the apple crop ( Malus domestica ) in central New York, Vermont, and southern Quebec. Climate extremes now rank with the costliest threats to agriculture, but their role in forest recovery from diebacks that are happening globally is unknown for lack of tree fecundity estimates in forests. Tolerance of climate extremes could depend on past exposure but constrained by phylogenetic conservatism. We report a continental scale analysis of climate extremes and forest fecundity across North America and Europe showing that responses to late freeze and drought are happening now. Species differences are not explained by the traits typically included in ecological studies and they are weakly associated with phylogeny. Late freeze, that is, freezing temperatures that follow the onset of flower development in spring, is shown to be “normal” in North America, but not Europe, potentially explaining failed seed production due to delayed onset and the resultant shorter growing period by North American transplants dating back at least to the 18th century. Drought has thus far had the greatest impacts in dry forested regions, but here too, species differences are not explained by traditional trait values. If responses have been buffered from drought and late freeze by past exposure, acclimation and local adaptation prove inadequate as extremes intensify.

Stomatal closure prevents significant water losses during drought events. Yet, leaves are not perfectly hermetic and dehydration ensues through residual water losses, known as minimum conductance (gmin), which is highly relevant since it informs on the water depletion dynamics under stress. We measured gmin on 101 species spanning phylogenetic and ecological diversities, from ferns to flowering plants. Sampling also included different growth forms and life cycles from annual herbs to longevous trees. We used stomatal measurements to estimate operational (gth op) and maximum (gth max) conductances. Minimum water conductance is highly variable across species, and gmin shows a weak phylogenetic signal across vascular plants. Annual herbaceous plants have greater water loss than woody plants, and deciduous species showed higher gmin rates than evergreen species. Relationships of gmin with gth op and gth max were weak, revealing the lack of a clear tradeoff between maximum potential conductance efficiency and water retention. We found reduced water expenses in species occupying hotter and more seasonal environments. This study integrates gmin in leaf economics, where long-lived leaves show higher capabilities to retain water under stress. We provide important information in evolutionary physiology to understand the water loss dynamics under drought stress across clades of vascular plants.

Late spring frosts (LSF) have substantial ecological and economic impacts in the temperate and boreal zones. Yet, the effects of climate warming on the frequency (i.e., probability of LSF in a given year, in %) and extent (i.e., percentage of trees in a population damaged by a given LSF event) of LSF damage remain underexplored. Here, extending a budburst model that accounts for within-population variability, we developed and evaluated a new model of LSF damage occurrence and extent using 1,220 observations of LSF damage to newly emerged leaves from 304 oak populations in France (1997-2021). Our model simulations reveal that overall, French oak populations are, over time, less exposed to LSF amid ongoing climate change. We observed an overall decline in the frequency (-0.22 % per year) and extent (-0.34 % per year) of LSF damage in French oak populations over the past six decades (1961-2021). These trends are largely driven by the temporal advance of both the last spring frost day and budburst dates, with the last spring frost day advancing at a slightly faster rate (-0.28 days per year) than budburst (-0.21 days per year). This temporal mismatch explains why, contrary to the common assumption that earlier budburst increases frost risk, earlier budburst was in fact associated with a lower frequency of LSF damage. Nevertheless, considerable geographical variability emerged, with declines in damage frequency being more pronounced in continental regions, whereas declines in damage extent were more pronounced in coastal regions. Our findings underscore the importance of considering both LSF frequency and extent when assessing frost risks in a warming climate, offering a comprehensive framework for future ecological and economic evaluations of LSF impacts.

To survive climate change, forest trees will have to shift seed production poleward. However, warming will not stimulate tree fecundity in the north if it is limited by other habitat variables. We evaluated the responses of tree fecundity to climate change for 292 tree species in North America and Europe, using response velocity, defined as (climate sensitivity) × (climate-change rate). The sensitivities to climate were estimated for each species and combined with rates of climate change to quantify how temperature, moisture deficits, and late freeze are influencing biogeographic shifts in tree reproduction. The results show that moisture deficit and late freeze, not annual temperature, drive changing seed production. Unlike annual temperature, which is increasing generally, change in these climate variables is not driving poleward shifts in seed production. These findings do not challenge the expectation that forests might eventually shift poleward. Rather, they show why current efforts offer divergent interpretations. The changes happening now are not consistent with annual temperature trends. As warming continues, fecundity changes can best be anticipated from temperature interactions with precipitation and extremes that impact flowering and fruiting in winter and spring.

As drought events become more severe, understanding drought-tolerance traits in crops is vital for food security. Brassica oleracea L., a widely consumed vegetable (including cabbage, cauliflower, and broccoli), has seen a rise in cultivation, but the response of different genotypes to drought remains poorly studied. Here, we investigated the mechanisms underlying drought resistance in seven genotypes of B. oleracea (two grandparental lines and five F2 genotypes) by examining stem anatomical and hydraulic traits, as well as stomatal characteristics, leaf dimensions, and stomatal regulation under drought conditions. Detailed measurements of stem anatomical traits were obtained using light and electron microscopy, while stem hydraulic vulnerability was assessed using the Optical Vulnerability method. The seven genotypes displayed contrasting water-use and hydraulic strategies. Under well-watered conditions, initial stomatal conductance (gsΨ0) showed considerable variation independent of their leaf and stomata size and density, while the leaf water potential at 90% stomatal closure (Ψgs90) was similar among genotypes. Stomatal safety margins (SSM) were positive across genotypes, confirming coordination between stomatal regulation and xylem hydraulics, and mainly driven by stem P50, which was approximately 1 MPa lower in the most tolerant genotype. Embolism resistance was best explained by the thickness of the intervessel pit membrane, highlighting the importance of including anatomical traits in plant drought tolerance studies. To conclude, the intraspecific variation observed in anatomical, hydraulic and stomatal traits among our B. oleracea genotypes indicates potential adaptability to drought and suggests that more drought-tolerant genotypes could be identified within this crop species through targeted, human-mediated crosses.

BACKGROUND AND AIMS:Photosynthetic tissues of terrestrial plants require a continuous water supply to avoid desiccation while allowing CO2 to diffuse to chloroplasts. This imposes a functional coordination between water-conducting tissues and stomatal regulation. What is the anatomical basis of this coordination? METHODS:We assessed anatomical traits of stomata, trichomes, and xylem vessels as well as branch allocation patterns in 30 olive varieties grown under common environmental conditions in Southern France. Theoretical conductances of leaves and stems were calculated and compared to maximal leaf conductance measured in the field. We also tested whether wood density could serve as a proxy for stem conductance. RESULTS:Stomata, trichomes, and xylem vessels showed positive covariation in size, but not in density. Consequently, no coordination was observed between the total stem vessel area and the total stomatal pore area, nor between the theoretical conductances of stems and leaves. However, we found a positive association between the stomatal pore area and the lumen area when values were scaled at the branch level. Maximal leaf conductance was negatively related to trichome area, but not to theoretical stomatal conductance. Wood density correlated negatively with the lumen fraction in stems, but not with stem conductance. CONCLUSIONS:We conclude that coordination between water transport and loss likely occurs at the branch level, involving allocation patterns beyond anatomy, and that the role of trichomes in water regulation deserves further study in pubescent-leaved species.

Abstract Intraspecific variability in hydraulic and morphological traits may alter projections of forest vulnerability to climate change. Recognizing multiscale nature is essential, as trait relationships within populations may not reflect those across the species’ range. We assessed variability in 11 traits in 10 natural populations of Abies alba across environmental and phylogeographic gradients to evaluate potential impacts on drought vulnerability. We quantified trait variation within and among populations, tested effects of climate, phylogeny and local factors, and evaluated trait coordination across scales. Hydraulic safety traits and wood density showed low variability, indicating strong constraints. In contrast, water-use and efficiency traits were highly variable. Aridity influenced several traits, but reduced variance was detected only for leaf residual conductance and succulence consisted with stabilising selection. Trait coordination was weak within populations than among populations. Overall, A. alba combines constrained hydraulic safety traits with variable water use traits. This may buffer drought impacts but limits shifts in hydraulic safety margins, potentially increasing risks of hydraulic failure, especially in humid environments where populations lack the capacity to tolerate prolonged dry periods. Thus, our findings highlight the need to account for scale-dependent trait variability, coordination, and their underlying drivers when predicting species’ adaptive capacity to drought.

Climate change is increasing the severity and frequency of heatwave and drought events, affecting forest community composition through changes in the mediating microclimatic effect of dominant adult trees. Predicting these changes requires understanding how temperature interacts with biotic factors, such as canopy-mediated facilitation and competition, during forest regeneration. Here, we investigate how the forest canopy modifies temperature- and light-related constraints on sapling survival along an elevation gradient. We conducted a reciprocal transplant experiment using saplings of Fagus sylvatica and Quercus petraea along an elevation-driven temperature gradient (100-1600 m) in the French Pyrenees. Saplings were planted under two tree-neighbour conditions, under forest canopy and in open gaps, allowing analysis of the effects of temperature, canopy cover and their interactions on survival. The experimentally derived survival-temperature relationships were then used to generate spatial projections of canopy effects on sapling survival under a warming scenario. Sapling survival declined with increasing elevation for both species, but responses differed between canopy conditions and species. Q. petraea survival decreased sharply under canopy at high elevation, whereas F. sylvatica showed reduced survival in open gaps at the lowest elevation. These patterns reflect contrasting biotic interactions along the thermal gradient: canopy competition constrains Q. petraea at high elevations due to heat limitation, while canopy facilitation benefits F. sylvatica at the lowest elevation by buffering heat and vapour pressure deficit. Spatial projections under climate warming suggested improved canopy effect for both species at lower elevations, whereas at higher elevations Q. petraea benefits from canopy, but F. sylvatica experiences degraded effects. Our findings highlight the key role of strain-the degree to which local conditions deviate from a species' physiological optimum-in shaping competitive and facilitative interactions. While environmental stress sets the external constraint, species strain determines whether the canopy cover functions as a buffer or an inhibitor. Synthesis. By altering microclimatic conditions, the tree canopy can either alleviate or intensify strain depending on the species, thus influencing sapling survival and establishment along thermal gradients. Our results underscore the need to explicitly account for biotic interactions in models of species distribution and forest regeneration under climate warming. Le changement climatique accentue la fr & eacute;quence et l'amplitude des vagues de chaleur et des s & eacute;cheresses, impactant ainsi la composition des communaut & eacute;s foresti & egrave;res par le biais de changements dans l'effet mod & eacute;rateur exerc & eacute; par la canop & eacute;e sur le microclimat du sous-bois. Pour pr & eacute;dire ces changements, il est. n & eacute;cessaire de comprendre comment la temp & eacute;rature interagit avec les interactions biotiques, telles que la facilitation et la comp & eacute;tition exerc & eacute;e par la canop & eacute;e, au cours de la phase de r & eacute;g & eacute;n & eacute;ration foresti & egrave;re. Nous & eacute;tudions ici comment le couvert forestier modifie les contraintes li & eacute;es & agrave; la temp & eacute;rature et & agrave; la lumi & egrave;re sur la survie des jeunes arbres le long d'un gradient altitudinal. Nous avons r & eacute;alis & eacute; une exp & eacute;rience de transplantation r & eacute;ciproque de jeunes plants de Fagus sylvatica (h & ecirc;tre commun) et de Quercus petraea (ch & ecirc;ne sessile) le long d'un gradient thermique altitudinal (100-1600 m) dans les Pyr & eacute;n & eacute;es fran & ccedil;aises. Les jeunes arbres ont & eacute;t & eacute; plant & eacute;s dans deux conditions contrast & eacute;es, sous couvert forestier et en clairi & egrave;re, permettant d'analyser les effets de la temp & eacute;rature, du couvert forestier et de leurs interactions sur leur survie. Les relations entre survie et temp & eacute;rature obtenues exp & eacute;rimentalement ont ensuite & eacute;t & eacute; utilis & eacute;es pour projeter spatialement les effets du couvert forestier sur la survie des jeunes arbres dans un contexte de r & eacute;chauffement climatique. Le taux de survie des jeunes arbres a diminu & eacute; avec l'altitude pour les deux esp & egrave;ces, mais les r & eacute;ponses ont vari & eacute; selon les conditions de couvert forestier et l'esp & egrave;ce. Le taux de survie de Q. petraea a chut & eacute; sous le couvert forestier aux altitudes les plus & eacute;lev & eacute;es, tandis que celui de F. sylvatica a diminu & eacute; dans les clairi & egrave;res & agrave; la plus basse altitude. Ces tendances refl & egrave;tent des interactions biotiques contrast & eacute;es le long du gradient thermique: & agrave; haute altitude, la comp & eacute;tition sous la canop & eacute;e limite la survie de Q. petraea en raison de contraintes thermiques, tandis que l'effet de facilitation du couvert forestier profite & agrave; F. sylvatica aux altitudes les plus basses en att & eacute;nuant la chaleur et le d & eacute;ficit de pression de vapeur. Les projections spatiales sous un sc & eacute;nario de r & eacute;chauffement climatique sugg & egrave;rent un effet plus favorable de la canop & eacute;e pour les deux esp & egrave;ces aux basses altitudes, alors qu'aux altitudes les plus & eacute;lev & eacute;es, la canop & eacute;e favorise Q. petraea mais affecte n & eacute;gativement F. sylvatica. Nos r & eacute;sultats mettent en & eacute;vidence le r & ocirc;le cl & eacute; du strain, le degr & eacute; de divergence entre les conditions locales et l'optimum physiologique d'une esp & egrave;ce, dans la d & eacute;termination des interactions comp & eacute;titives et facilitatrices. Tandis que le stress environnemental impose une contrainte externe, le strain d & eacute;termine si le couvert forestier agit comme un facteur de faci En modifiant les conditions microclimatiques, le couvert forestier peut att & eacute;nuer ou renforcer les contraintes auxquelles sont soumises les esp & egrave;ces, influen & ccedil;ant ainsi la survie et l'& eacute;tablissement des jeunes arbres le long des gradients thermiques. Nos r & eacute;sultats soulignent la n & eacute;cessit & eacute; d'int & eacute;grer explicitement les interactions biotiques dans les mod & egrave;les de distribution des esp & egrave;ces et de r & eacute;g & eacute;n & eacute;ration foresti & egrave;re dans un contexte de r & eacute;chauffement climatique.

The island rule predicts gigantism or dwarfism in body size of island species relative to their mainland counterparts. However, whether other functional traits shift and whether trait-trait associations on islands differ between species and community levels remains unclear. We measured 13 carbon- and water-related functional traits in 37 shared tree species across 35 eastern Chinese islands and 66 nearby mainland plots. We examined species-level trait value shifts and associations under the island rule and compared trait associations between species and communities. Most size-related, wood-anatomical, and hydraulic traits shifted on islands, with large values decreasing and small values increasing; yet, their associations remained stable, aligning with the global trait spectrum and trait-trait coevolution. This stability, despite trait value shifts, suggests evolutionary integration of functional strategies. By contrast, island community-scale trait associations diverged from shared species-level patterns and sometimes reversed, such as positive relationships between wood density and resource-acquisitive traits. Community-level trait associations were stronger on islands, likely reflecting constrained environmental filtering and migration limitation. These contrasting patterns suggest that dominant species can restructure trait associations at the community level, with implications for ecosystem functioning and carbon storage, thereby advancing understanding of plant trait strategies in island systems.

Process-based models that mechanistically represent water-carbon balances in the atmosphere-soil-plant continuum are an attractive tool for monitoring live fuel moisture content (LFMC) dynamics, a key variable when assessing fire danger. However, their application as operational tools to assess near-term wildfire danger at regional scale faces important challenges. Here, we explored key sources of prediction uncertainty in process-based modeling of LFMC. We applied the SurEau-ECOS model of plant hydraulics embedded within the MEDFATE modeling framework to assess how the accuracy of LFMC predictions was influenced by input data sources, by the availability of species-specific plant traits and by the level of mechanistic detail used to model water content of plant tissues. A lack of accurate data describing soil physical properties compromises the application of process-based models for predicting LFMC. Nonetheless, using global meteorological and vegetation data allows for successful regional-scale applications. Fully mechanistic approaches that model LFMC from plant water status using ecophysiological knowledge yield more accurate predictions. However, when reliable plant traits are lacking, semimechanistic approaches based on empirical equations offer a robust alternative. Overall, addressing the sources of uncertainty highlighted here could pave the way for developing operational tools to forecast near-term wildfire danger through process-based modeling of LFMC dynamics.

Arbuscular mycorrhizal fungi (AMF) contribute to plant nutrient and water uptake via their extraradical hyphal networks. However, in situ methodologies to quantify architectural and morphological traits of these networks in soil are largely lacking, limiting our understanding of AMF-mediated resource transport. Using synchrotron-based X-ray computed microtomography (micro-CT), we established a workflow to cultivate, noninvasively image, and quantitatively analyze AMF hyphosphere and rhizosphere structures in the interaggregate space across two soil textures and biological contexts. We developed a pipeline for quantitative three-dimensional (3D) assessment of key architectural and morphological traits including structure counts, hyphal length, branching frequency, volume, and surface area. Our method further permits (1) measurement of AMF-soil and AMF-root interface areas and (2) microscale quantification of pore space occupancy by AMF. Micro-CT offers a tool for noninvasively visualizing AMF in air-filled soil pore space. We outline how such quantitative 3D information can be incorporated into image-based and functional-structural soil-plant models, thereby supporting a better mechanistic understanding of AMF-mediated processes in soils and plants.

Xylem embolism persists long after drought has ended and should thus have a lasting effect on plants' ability to recover from severe drought. This assumption has rarely been tested, as quantifying the amount of embolism in individual intact trees is difficult, and following recovery would require long-term monitoring, specifically when studying trees. Our goal was to test the effect of embolism on recovery from drought in a broad population. We exposed 210 carob seedlings (Ceratonia siliqua), a species with high resistance to embolism, to a gradient of drought durations and monitored their ability to recover over five months with respect to their embolism level (assessed using micro-computed tomography). Seedlings that suffered 38% embolism had a 50% chance of dying. In the surviving seedlings, stomatal conductance was still inhibited a month after rehydration (ie 82% lower than in the irrigated control, even in plants that sustained only 10% embolism) but recovered to pre-drought levels after 5 months, regardless of the embolism levels. The hydraulic limitations were mostly noticed in the canopy size, as 5 months after rehydration, canopy area and the ability to resprout were strongly correlated with embolism level sustained during drought (but not with water potential). Our results suggest that embolism can be fatal even at levels below 50% and that maintaining the integrity of the hydraulic system is critical for rapid drought recovery.

Summary Plant ecological and evolutionary strategies are shaped by interactions between phylogenetic history and environmental constraints, resulting in leaf and stomatal traits. However, traditional trait-based and phylogenetic approaches often fail to fully explain biochemical mechanisms underlying ecological strategies, particularly for leaf and stomatal traits. Plant metabolomes integrate genetic, physiological, and environmental information and therefore represent a promising intermediate phenotype for investigating links between biochemical diversity, functional traits, and evolutionary patterns. We analysed metabolomic profiles from 74 plant species with various growth forms and ecological types. Using machine learning approaches, we explored whether metabolic variation could predict plant functional divisions, growth forms and phenological types, but also physiological traits related to drought resistance. Metabolomic data contained structured information associated with variation in plant functional traits, ecological strategies, and phylogenetic relationships. Machine learning models identified with high accuracy distinct metabolic signatures linked to differences among plant functional divisions, growth forms, phenology, and trait values. Our study demonstrates that predictive metabolomics provides a powerful and integrative framework to investigate plant ecological and evolutionary strategies. By linking biochemical diversity with plant phylogeny, and ecophysiological traits across multiple species, this approach offers new opportunities to explore the mechanistic basis of plant evolution.

### Competing Interest Statement The authors have declared no competing interest.

Leaf water loss after stomatal closure is key to understanding the effects of prolonged drought on vegetation. It is therefore important to accurately quantify such water losses to improve physiology-based models of drought-induced plant mortality. We measured water loss of detached leaves continuously during dehydration in nine woody angiosperm species. We computed minimum leaf conductance (gmin) at different water potential thresholds along a sequence of physiological function losses, spanning from turgor loss point to hydraulic failure. A mechanistic model evaluated the impact of different gmin estimations on the time to hydraulic failure (THF). Residual conductance is not steady and decreases continuously at varying rates across species during the entire dehydration process, even after correcting for leaf shrinkage and vapor pressure deficit shifts. Different estimations of gmin had a significant impact on the THF predicted by the model, especially for drought-resistant species. We demonstrate that residual conductance is variable during dehydration, and thus, it is important to use physiological or water status boundaries for its estimation in order to determine distinct gmin values of water loss. We describe an accurate, repeatable and open-source methodology to estimate gmin. Such methodology could enhance models of plant mortality under drought.

The size of a tree and its crown are key drivers of its ability to intercept light. These characteristics are also constrained by biomechanical and hydraulic limitations, leading to tremendous variations in crown size and tree height between species. To date, there is no consensus on how covariation between crown characteristics (i.e. tree height, crown diameter and relative crown depth) control the spectrum of crown sizes, nor on the species characteristics (i.e. species climate niche or functional traits) that govern their variation. Using species‐specific allometric relationships accounting for local competition data on individual crown characteristics of tree species across Europe and North America, we showed that the spectrum of tree crown size is mainly driven by two independent axes: one correlating tree height with relative crown depth, and another linking crown diameter to crown volume. While both angiosperms and gymnosperms exhibited similar patterns of covariation for these crown characteristics, distinct ecological strategies emerged for maximum tree height. We also highlighted that shade‐tolerant species were characterized by wider and deeper crowns and showed reduced sensitivity to local competition in relative crown depth, indicating enhanced competitive resilience. Finally, simulations of light interception in virtual stands performed using a ray tracing algorithm highlighted the importance of different crown characteristics in various competitive contexts. Tree height was the primary determinant of light interception in low competitive contexts, common in early successional stages, whereas relative crown depth and crown diameter played a more significant role in high‐competitive contexts typical of late succession. These findings suggest that shade‐tolerant species, with their deeper crowns, are better equipped to intercept light under competition and ultimately emphasize the ability of shade tolerance to improve the realism of forest dynamic models as it can be used as an indicator of crown size and its response to competition.

With increasing drought events worldwide, crop breeding must focus on drought resistance to maintain crop yields. To ensure a high level of gas exchange and growth, plants need to maintain the integrity of their vascular system under drought conditions. While the impact of drought-induced vascular damage on tree species is well-documented, its effect on the yield of annual crops like soybean (Glycine max (L.) Merrill) remains unknown. We investigated xylem vulnerability to embolism of ten soybean cultivars with contrasting phylogenetic origins and phenology using the optical technique. With X-ray micro-tomography, we assessed xylem vulnerability across the plant vascular pathway to quantify the vulnerability segmentation. Our results revealed that soybean is moderately vulnerable to xylem embolism (mean leaf P50 = -1.85 MPa), with a significant Intraspecific variability with a difference of 1 MPa between the most extreme cultivars. Cultivars with higher leaf embolism resistance maintained higher yields in the field, particularly during dry years, highlighting the critical role of xylem hydraulic failure during drought in crop yield. This study provides new insights into the importance of hydraulic traits underlying drought tolerance in soybeans and their incorporation into breeding programmes for embolism resistance to improve yield resilience.

Phenotypic plasticity in associations enables higher resource use efficiency and niche complementarity. This study aims to (1) examine the phenotypic plasticity of soybean functional traits to relay-cropping under drought conditions and (2) evaluate different soybean genotypes and their interactions. A preliminary investigation comprised field experiments in micro-plots in non-irrigated conditions performed for two years (2022 and 2023) to examine trait responses to barley relay-cropping for four soybean genotypes compared to sole crops. In 2022, we experienced a record-setting drought that highly impacted the soybean yield. Ten above- and below-ground functional traits related to light interception, resource allocation and soil exploration were measured at barley harvest and one month after, at the reproductive stage. Our results showed that relay-cropping can exacerbate drought stress and, consequently, competition for water between crops. This led to phenological delay and growth abortion. However, root functional traits, such as lateral root mean angle, specific root length and root nodules densities, were higher in relay-cropping, demonstrating plant response to water stress and competition. Soybean genotypes showed a plasticity of root functional traits to respond to competition and water stress. Agricultural practices (timely irrigation and/or adapting relay-cropping design) could modulate water stress, and appropriate cultivars would allow relay-cropped soybeans to sustain temporally dry conditions.